In such systems that use resonators, the signal to be measured is the resonant frequency of the nanoresonator, in fact depending directly on the weight of matter deposited on the resonator, a greater weight generating a lower resonant frequency.
To measure the resonant frequency, it has already been proposed to use a self-oscillating circuit comprising a closed oscillation loop incorporating the resonator. FIG. 1 schematically represents such a self-oscillating circuit. The self-oscillating loop comprises the resonator NMS in series with an amplification and phase-shifting subsystem, and a feedback between the output of the subsystem and an excitation input of the resonator. The amplification subsystem adds a gain by an amplifier AMP and a phase shift by a phase shifter DPH; it makes it possible to ensure natural oscillation conditions (open loop gain greater than or equal in modulus to 1 for a loop phase shift that is a multiple of 2π). The oscillation frequency is the natural mechanical resonant frequency of the resonator NMS. It is measured at the end of the amplification subsystem by a frequency meter FMTR. The latter can operate, for example, on the principle of counting pulses of a reference clock CLK that has a frequency very much greater than the oscillation frequency. The analogue or digital output S of the frequency meter supplies a measurement of the natural resonant frequency of the resonator. This solution makes it possible to produce circuits with little bulk, which is important notably in the case where the aim is to produce a network comprising a large number of nanoresonators. However, since there is a wide technological dispersion in the resonators and the components of the amplification subsystem, it is difficult to guarantee a priori that gain and phase conditions will be obtained that allow for a spontaneous natural oscillation at the resonant frequency.
Phase-locked loop (PLL loop) circuits have also been proposed, such as the one schematically represented in FIG. 2. The circuit also comprises a nanoresonator NMS in series with an amplifier AMP, a voltage-controlled oscillator (VCO) or a digitally-controlled oscillator (DCO) for exciting the resonator (NMS), a phase comparator CMPH, and a subtractor SUB for subtracting from the output of the phase comparator a value (modulo 2π) which represents the natural phase shift ΔΦref introduced by the resonator and the amplifier at the resonant frequency. A low-pass filter FLTR is inserted between the output of the phase comparator and a control input of the oscillator to ensure the stability of the locked loop.
The value ΔΦref is a phase-shift value measured by calibration by having the resonator and the amplifier operate in open loop mode at the resonant frequency and by observing the phase shift between the excitation signal of the resonator and the output of the amplifier.
The circuit is automatically locked on to the frequency for which the phase shift between the inputs of the phase comparator is equal to ΔΦref; this frequency is the natural resonant frequency of the resonator. In practice, in closed loop mode, the output of the phase comparator represents the phase shift of the resonator and of the amplifier. If it is not equal to ΔΦref, the control voltage of the oscillator VCO is adjusted until it becomes equal to ΔΦref, the phase shift corresponding to resonance. The measurement of the resonant frequency is then done by measuring the control voltage Vout of the oscillator, this voltage representing the oscillation frequency of the oscillator. This solution with PLL loop requires a preliminary calibration to know the phase shift ΔΦref at resonance.
The circuits using such a phase-locked loop consume more current than the circuits that operate in natural oscillation mode. Also, they are bulky.